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Men and women have close and intimate relationships, which change over the decades. Around the age of 50, a man might notice that the love of his life doesn't ring him as much as she used to. The guy is correct, and yes, there is someone else in her life. But that someone is not another man — it's probably his daughter.

Most human societies are monogamous, and our families span several generations. So as time goes by, we change the emotional 'investment strategies' that we use with our nearest and dearest. For example, in a woman, menopause is the end of reproduction for her. But she might have a daughter who may well reproduce.

How can you get a snapshot into people's lives and relationships? A study by Dr Palchykov and colleagues got their snapshot from an unnamed European country. They analysed the phone records of one phone provider that had 6.8 million subscribers.

However, this study specifically wanted to explore relationships. So it looked only at phone calls and text messages between pairs of phone numbers that regularly returned each other's phone calls or text messages. This left them with 3.2 million subscribers — 1.8 million male, 1.4 million female. Over a seven-month window, these subscribers made 1.95 billion phone calls, and 489 million text messages. The data had been filtered by the phone provider so that the only information given to the researchers was the subscriber's age and gender, and the age and gender of people they communicated with.

The researchers analysed the data to find each subscriber's first and second 'best friends' — the people they called and texted most often.

When the researchers looked at people in early adulthood, they found that both the men and women focused their attention mostly on one member of the opposite gender (as measured by how many phone calls and texts they made). Presumably, this person was their beloved. The women began focusing their attention on their special person at a younger age than men did — 18 years for the women, and 22 for the men.

The intensity of the relationship (or at least the number of phone calls and texts) peaked at 27 for the women and 32 for the men. So in this unnamed country, these are the peak ages for preferring your best friend to be of the opposite gender.

Interestingly, the women maintained this intensity of communication for longer than the men did — 14 years for women and seven for the men. (Again, this was measured by the number of phone calls and texts made by the men and women.)

The authors write that: "Women are more focused on opposite-sex relationships than men are during the reproductively active period of their lives, suggesting they invest more heavily in creating and maintaining pairbonds than men do".

From the viewpoint of evolutionary biology, this makes sense for women wanting to have children.

But when the women reach their 40s, their first best friend changes from being a similar-aged male — to a female who is about 25 years younger.

We can't be sure without access to more of the phone company data, but almost certainly, this was the adult woman's daughter. This transition from the older male to the younger female was relatively smooth. The presumed mother-daughter relationship increased in intensity over the next 15 years, reaching a peak as the older woman reached the age of 60.

Probably this increased communication is related to the arrival of grandchildren. The new grandmother now switches her time and energy into nurturing her relationship with her daugher and her grandchildren. Mother-daughter relationships are especially important in structuring human social relationships.

But what about the man who used to be that woman's first best friend? Well, now he is her second best friend (after their daughter), but at least he stays that way until "death do them part". And he reciprocates by still keeping her as his first best friend.

So around 70 years of age, both genders are in synchrony — as they each prefer to have a female as their new best friend for ever (their BFF or bestie).